A permanent magnet synchronous motor (PMSM) drive system is a new drive system that takes PMSM as a control object and controls the rotational speed and torque of the PMSM by means of frequency modulation and voltage modulation. Compared with a conventional alternating-current drive system taking an asynchronous motor as a control object, the PMSM drive system has advantages such as simple structure, high power density, large output torque at low speeds, high efficiency and convenient maintenance, and is overtaking an asynchronous motor alternating-current drive system as a future mainstream.
In a PMSM drive system, a direct-axis current of the PMSM directly affects distribution of current angles, a power factor of the PMSM during operation, reactive heat losses of the system, and weak magnetic properties of the PMSM at high speeds. More importantly, the permanent demagnetization of permanent magnetic materials for a rotor depends on the magnitude of the direct-axis current, and the permanent magnet will generate irreversible demagnetization in a case that the direct-axis current exceeds a demagnetization current of the permanent magnet. Thus, the protection for direct-axis current component of the PMSM is particularly important.
In conventional technology, from the perspective of design for the permanent magnet motor, permanent failure of the permanent magnet due to an excessive direct-axis current is protected through limiting a maximum operating point of the permanent magnet.
Referring to FIG. 1, a graph of a demagnetization segment of a B-H curve for the permanent magnet is shown.
FIG. 1 shows the demagnetization segment of the B-H curve for the permanent magnet. The permanent magnet operates under this segment in case of a motor in operating condition. The permanent magnet at no load operates at point A in case of a motor at no load, for an air gap causes demagnetization of the permanent magnet. An armature corresponding to the direct-axis current will generate further demagnetization effect in case of a motor at loads, i.e., the permanent magnet at maximum load operates at point B. The worst demagnetization effect occurs in case of a motor with short circuits, such as point C. The permanent magnet will generate reversible demagnetization if the point C is below the inflection point. Therefore, the operating point corresponding to the worst demagnetization caused by the short circuit of the permanent magnet motor is designed above the inflection point of the permanent magnet or same as the inflection point of magnetic field line of the permanent magnet, to avoid permanent magnetism-loss of the permanent magnet due to large demagnetization effect of the armature corresponding to the direct-axis.
From the perspective of motor control strategy, there is no article that describes the protection for the direct-axis current nowadays in the world. Only in documents that describe a vector control strategy, a currently given value is limited to meet the need of control, and the main implementation is to obtain a quadrature-axis current iq_ref through a proportional integral (PI) regulator for torque or rotational speed and obtain a given direct-axis current id_ref through the maximum torque-current ratio, with a restriction: |id_ref|<id_MAX.
From the perspective of motor design, the operating point of the permanent magnet with maximum direct-axis operating current is designed to be same as the inflection point or above the inflection point, to avoid permanent failure of the permanent magnet of the rotor due to a large direct-axis current component. However, the direct-axis current of the motor during actual operation is unknown, permanent failure of the permanent magnet will be caused if the actual direct-axis current is too large due to a system crash.
From the perspective of control, a given direct-axis current component is limited during the control only for preventing the given current exceeding a range of safe operation of the motor, without protection for the actual direct-axis current of motor and for the current of next moment generated by the motor based on a voltage at the present moment. Magnetism-loss of the permanent magnet motor will still be caused if the current of the motor is out of control.
In conventional technology, in the PMSM drive system, only a currently given direct-axis current is limited for protection, but the direct-axis current of next moment caused by the voltage at the present moment is not pre-calculated. If the direct-axis current of next moment is high, a too large reactive component, low power factor and large loss of the PMSM drive system will be caused, which leads to a catastrophic failure of the system.